This invention relates to nuclear reactors of the kind having a core structure comprised of closely packed parallel fuel element assemblies. Such fuel element assemblies may be of the type in which a plurality of elongate fuel pins are arranged parallel to one another and spaced apart within an outer tubular casing. Close packing of such fuel element assemblies is to be found in reactor cores without fixed moderator, that is to say in fast reactor cores and in reactor cores cooled by moderating liquids, such as in pressurised and boiling water reactors.
In a typical sodium cooled fast reactor core structure the fuel element assemblies are supported from a bottom core support structure or diagrid. Entry of sodium coolant into the fuel element assemblies is at their lower ends that is, in the region of the diagrid. Coolant flow is through the outer tubular casings of the fuel assemblies over the fuel pins contained therein, heat being extracted from the fuel pins by the coolant flowing through the casings. The rate of coolant flow through the fuel element assemblies is controlled by gagging means located within the lower ends of the casings of the fuel element assemblies.
In a nuclear reactor core structure the neutron flux distribution varies across the width of the core structure; generally the neutron flux is a maximum at the centre of the core structure falling to a lesser value at the periphery.
Materials such as stainless steel, which are used for the manufacture of fuel element assemblies of the kind described, are subject to the phenomenon of irradiation induced voidage growth. This phenomenon entails physical growth of material under neutron irradiation, the degree of growth being dependent on the intensity of the neutron flux. Fuel element assemblies at certain positions in the core structure will be subject to a neutron flux gradient. Because of the progressive reduction of neutron flux towards the periphery of the core structure, a fuel element assembly will be subject to a lower neutron flux on its side towards the outside of the core structure and will be subject to a higher neutron flux on its side towards the centre of the core structure. Thus the two sides of the casing of the fuel element assembly towards the outside and towards the inside of the core structure will be subjected to a differential growth. The side of the casing of the fuel element assembly towards the centre of the core structure will be subject to a greater growth than the side towards the outside of the core structure. This differential growth will induce bowing in the fuel element assemblies which can give rise to difficulties in operation of the reactor. For example the bowing can cause difficulties in removal of the fuel element assemblies from the core structure during a refuelling operation.
Irradiation induced voidage growth is known to be temperature dependent. Lowering of the operating temperature of the casings of the fuel element assemblies can result in a significant reduction of the degree of bowing which occurs.
One method of reducing the casing temperature of fuel element assemblies is by under gagging the fuel element assemblies so as to increase the rate of coolant flow therethrough. Increased coolant flow means that the coolant temperature and hence the temperature of the casings of the fuel element assemblies is decreased for the same rate of heat extraction. However this procedure has disadvantages because the theoretical efficiency of the plant is reduced by the decrease of coolant outlet temperature that results.